The investigative journalism news outlet ProPublica just published an article exposing the dangers posed by hundreds of thousands of abandoned oil and gas wells across the country to the environment and the nation’s drinking water supplies. The article reports that energy companies and others have drilled some 12 million such holes across the U.S., with many left abandoned and unplugged providing “pathways for oil, gas or brine-laden water to contaminate groundwater supplies or to travel up to the surface.”

A 2009 draft report from Pennsylvania’s Department of Environmental Protection, for example, found that old gas-leaking wells “represent a threat to public health, safety and welfare, and is a potential threat of a fire or explosion.” The report details incidents of abandoned oil wells leaking gas into ground and water supplies; and causing sometimes fatal explosions when combustible gas levels build up inside homes. The problem is hardly isolated to Pennsylvania. ProPublica found that old oil and gas wells were contaminating water supplies in Texas, New York, Kentucky, Colorado and elsewhere.

One survey by the Interstate Oil & Gas Compact Commission cited in the story found that there are at least 50,000 of these orphaned wells (and likely a million more) nationwide causing environmental and harm to public health in a number of ways. The survey finds that: “Newer, deeper wells can pose both physical and environmental hazards, because hydrocarbons, salts, and ground water migrate. An unplugged well creates a conduit allowing these materials to mingle, either contaminating below ground aquifers and water wells or seeping to the surface to contaminate fields, waterways, or ponds. Beyond the contamination, surface seeps can accelerate the risk and ferocity of wild fires. As unplugged wells deteriorate over time, they can cave in on themselves or give way to unsuspecting animals and humans. Unplugged wells also might allow water and salt migration to contaminate petroleum reserves.” But plugging each well is not a cheap undertaking for states—and the original drillers are often impossible to track down. Costs range from $2,000 to $40,000 per well to plug, with most falling in the $6,000-$12,000 range.

At times, such abandoned wells represent major contamination issues for nearby residents—as when 40,000 people’s drinking water was contaminated by chlorides from leaking wells in Fort Knox, Kentucky, over 20 years ago. ProPublica reported that at the state’s current rate of plugging 250 to 300 affected wells per year, “it will take more than 40 years to work through the nearly 13,000 wells the state has identified so far.”

The problem of leaking wells is likely to grow even more serious in coming years as drillers search for gas via hydraulic fracturing, particularly in the oil-rich Marcellus Shale in Pennsylvania, New York and surrounding states.